I need to cook one item at 400 degrees for 90 minutes, and another at 325 degrees for 30 minutes. If I want to cook them together, what temperature should I use and how long should I cook each? Many thanks.

Anything that cooks at 400 for 90 minutes is probably going to need a long time to cool-- if I were you I'd do the 400 first, and drop the temperature and throw the second item in; turn the temp down on the oven, obviously, and use the oven door and the thermometer to drop the temperature quickly-- it can be done literally in seconds-- do 5-full-second increments and watch the temp inside the oven.

If you must do them at the same time, drop the temperature and leave the 400-degree item in there-- again, anything that can cope with an hour at 400 should tolerate a temperature drop of 75 degrees. I would hope.